Less than a Virus

25 October, 2009

We animals are susceptible to a bewildering array of infections. On the largest scale we can be infested by other animals. Worms, lice and other parasites can rather literally get under our skin and cause great harm. Smaller than the animals are bacterial infections. Single celled organisms that reproduce far more rapidly than our own cells, bacteria are often controlled by killing them with antibiotics, exploding their cell walls and leaving the remnants to be cleaned away by our immune system. Smaller again than bacteria are viruses. Not even truly alive, viruses are strands of DNA or RNA wrapped in a protein sheath. Unable to reproduce by themselves if left in the open they will often degrade into something that is harmless. But put them near one of our cells that they can attach onto and something else happens. They invade the host cell, often pulled in by the cell itself which sees the virus as something useful. Inside the cell the complex apparatus goes to work and ends up creating more of the virus, reproducing it as if it were part of the cell. Some viruses will then be pushed out of the cell which continues making more of the virus, in other cases the cell simply fills up with virus particles and explodes releasing the virus to continue infecting neighbouring cells.

Each of these three type of infection can be bad enough, but there is a fourth type the we have only begun to really understand since the 1980s. Creutzfeld-Jakob disease (CJD) is a rare inheritable disease that attacks the brain and is always fatal. It seems that a particular mutation can cause the malformed creation of prions, and these malformed prions cause damage to the brain. We all have prions in our brains, a protein called PrPC sits at the surface of our nerve cells and is believed to help with signalling or transmembrane transport. Those who have the defective gene for creating this protein end up with CJD instead. But this disease is not just inherited, it can be passed on as well. In Papua New Guinea people who took part in elaborate funeral rituals which included eating the brain of the deceased contracted a disease called Kuru which was strikingly similar to CJD. Experiments revealed that by injecting the malformed protein (PrPS) into an animal, the animal would develop a similar disease. So now an inheritable disorder was also caused by an infectious agent.

Comparisons of PrPC (the healthy protein we all have) and PrPS revealed something startling. There was no difference in the amino acid chain that made up these proteins, they were identical in makeup, but different in the way they folded. Proteins are made up of long strings of amino acids which fold into three-dimensional shapes, and these shapes determine their properties. By folding in a different way the PrPS becomes lethal. It also affects normal PrPC – for some reason the PrPC reforms itself into PrPS when it comes into contact with the PrPS. So by eating the brain of someone who has PrPS, or by having surgical instruments that have the PrPS on it stuck into you, you may end up having your normal proteins changed into something deadly.

In the UK in the 1980s, cattle were being destroyed to prevent the spread of BSE, another prion disease. In the 1990s a small number of people died of vCJD (a CJD variant) which was identified as having come from BSE infected beef. The ten people who died from vCJD were all very young (average age of 27) compared to classical CJD where the average age of onset is 65. This leads to the thought – what is the incubation period for vCJD? And how variable is it? In a worst case scenario, the people who have already died from vCJD represent an outlying anomaly and the full force of the disease has yet to reveal itself. The average length of time for CJD to express itself after someone is infected by contaminated surgical instruments is 15 years. It has been more than 15 years since the BSE was removed from the vast majority of british beef and the rate of vCJD has not spiked so we may be looking at a best case scenario where only a few hundred die of this disease.

In any case, proteins are not susceptible to our normal methods of sterilising (radiation and heat do not ‘kill’ them since they are not alive anyway) so unless we increase our understanding of prions we face a fatal disease, already within a number of people, with no hope of a cure. Doctor’s advice: don’t go eating anyone’s brain. At least it wont spread that way.